A cable gland hole is the thread outside diameter plus a small clearance — 20 mm thread, 20 to 20.2 mm hole, and the same margin holds across the range. That single rule covers metric and PG glands, because both are parallel threads that seal on the panel face rather than in the thread.
What the rule does not cover is everything that decides whether the gland actually seals after the hole is cut: how the hole was made, how thick the panel is, whether there is a flat face for the gasket, and how close the next hole is. This page gives the hole table first, then the four things that make a nominally correct hole fail.
| Thread | Hole (mm) | Thread | Hole (mm) | Thread | Hole (mm) |
|---|---|---|---|---|---|
| M14×1.5 | 14–14.2 | M28×1.5 | 28–28.2 | PG11 | 18.6–18.8 |
| M16×1.5 | 16–16.2 | M30×1.5 | 30–30.2 | PG13.5 | 20.4–20.6 |
| M18×1.5 | 18–18.2 | M32×1.5 | 32–32.2 | PG16 | 22.5–22.7 |
| M20×1.5 | 20–20.2 | M33×1.5 | 33–33.2 | PG19 | 25–25.2 |
| M22×1.5 | 22–22.2 | M36×1.5 | 36–36.2 | PG21 | 28.3–28.5 |
| M24×1.5 | 24–24.2 | M40×1.5 | 40–40.2 | PG25 | 32–32.2 |
| M25×1.5 | 25–25.2 | M42×1.5 | 42–42.2 | PG29 | 37–37.2 |
| — | — | M50×1.5 | 50–50.2 | PG36 | 47–47.2 |
| — | — | M63×1.5 | 63–63.2 | PG48 | 59.3–59.5 |
Hole dimensions are from our standard brass and stainless steel series, where the hole is specified as thread O.D. to O.D. + 0.2 mm. Small threads behave identically: PG7 is 12.5–12.7 mm and PG9 is 15.2–15.4 mm.
Three reasons, and none of them are cosmetic. The thread has a manufacturing tolerance and cannot be assumed to be exactly nominal. The gasket needs somewhere to sit without being squeezed out of position. And the gland has to go in without being driven in — a thread that is forced through a tight hole loses its coating on the way and starts life as a corrosion site.
The 0.2 mm band is deliberately narrow. Wider and the gasket has nothing to bear on at its outer edge; narrower and assembly becomes a press fit. When a supplier quotes a hole range without saying which thread it belongs to, the number is useless — 20.2 mm is a metric number, and the nearest PG thread to it is 20.4 mm, which will not go in. The thread size chart has the diameter for every size in both families if you need to cross-check.
Metric threads are the ones most likely to be cut into new panels, so they are also the ones most likely to be damaged by a rushed hole. Note that the hole grows in 1.5 mm steps with the thread pitch, which means a 20 mm hole saw cannot be used for anything other than M20 — there is no metric gland that shares that hole.
| Thread | Hole (mm) | Nominal step from previous | Common installation |
|---|---|---|---|
| M14×1.5 | 14–14.2 | Smallest standard body | Sensor and signal entries |
| M16×1.5 | 16–16.2 | 2 mm | Small control cable |
| M18×1.5 | 18–18.2 | 2 mm | Instrument cable |
| M20×1.5 | 20–20.2 | 2 mm | The default entry on most equipment |
| M22×1.5 | 22–22.2 | 2 mm | Power cable, smaller sizes |
| M24×1.5 | 24–24.2 | 2 mm | Where M25 will not fit |
| M25×1.5 | 25–25.2 | 1 mm | Power and motor cable |
| M28 / M30 | 28–28.2 / 30–30.2 | 2 mm | Larger power entries |
| M32×1.5 | 32–32.2 | 2 mm | Main feed on medium panels |
| M40 / M50 / M63 | 40–40.2 / 50–50.2 / 63–63.2 | Steps of 10 and 13 mm | Incoming supply, large cable |
PG holes are the ones that cannot be fixed later. They are sized in a sequence that has no relationship to whole millimetres — 18.6, 20.4, 22.5, 28.3, 59.3 — so the hole has to be made from the thread designation, never from “about 20 mm”. A PG11 needs 18.6 mm; drilling 18 mm and expecting it to take a PG11 produces a hole that is 0.6 mm tight on the thread and 0.2 mm under the top of the tolerance band.
| Thread | Hole (mm) | Nearest metric thread | Why the substitution fails |
|---|---|---|---|
| PG7 | 12.5–12.7 | None at this size | No metric equivalent in the series |
| PG9 | 15.2–15.4 | M16 (16–16.2) | PG9 hole is 0.8 mm smaller than the metric one |
| PG11 | 18.6–18.8 | M18 (18–18.2) | Metric hole is undersize for the PG thread |
| PG13.5 | 20.4–20.6 | M20 (20–20.2) | Metric hole is 0.4 mm undersize |
| PG16 | 22.5–22.7 | M22 (22–22.2) | Metric hole is undersize |
| PG19 | 25–25.2 | M25 (25–25.2) | Identical hole, different pitch in the thread |
| PG21 | 28.3–28.5 | M28 (28–28.2) | Metric hole is undersize |
| PG25 | 32–32.2 | M32 (32–32.2) | Identical hole, different pitch |
| PG29 | 37–37.2 | M37 (37–37.2) | Identical hole, different pitch |
| PG36 / PG42 / PG48 / PG63 | 47 / 54 / 59.3 / 72 | None close | No metric thread lands on these diameters |
PG19 and PG25 are the awkward pair. Their holes are identical to M25 and M32 respectively, so a metric gland will physically enter a PG hole and vice versa. It will start, bind within a turn or two, and the fitter will assume the thread is damaged. The hole is not wrong; the thread is. That distinction is worth teaching to whoever runs the punch.
The same thread needs two different holes depending on what it is going into, and confusing them is the most common drawing error on an enclosure.
| Panel arrangement | Hole to cut | What secures the gland | Failure if done wrong |
|---|---|---|---|
| Clearance hole through sheet or cast box | Thread O.D. + 0.2 mm | Locknut on the far side, or a threaded boss | Hole too large: gasket has no bearing face |
| Threaded (tapped) hole in a thick boss | Tapping drill for the thread | The thread itself | Clearance hole in thick metal: nothing to clamp against |
| Hole through a thin wall with no locknut | Thread O.D. + 0.2 mm | Nothing — the gland cannot clamp | Gland pulls through or cannot be tightened |
| Hole through a wall thicker than the thread length | Thread O.D. + 0.2 mm | Nothing — thread does not emerge | No locknut possible, gasket never compressed |
That last row is the one that ends in a rework. If the thread does not come out the far side, there is no way to put a locknut on it, and no way to pull the gland down onto the gasket. The gland will sit in the hole and look installed. It will not seal. Check the thread length against the wall before the hole is cut, not after.
Two holes measured at 20.1 mm can behave completely differently. A punched hole is round and has a slight roll on one side. A hole saw cuts a hole that is typically a little oversize and can wander if the pilot is not rigid. A hand drill with a step bit leaves a chamfer and, often, a burr on the inside face.
Deburr both faces, then check with a finger rather than an eye. If a fingernail catches, the gasket will too.
A gland gasket seals against a flat annulus around the hole. The hole can be perfect and the seal still fail because the surface it bears on is not flat, not wide enough, or not continuous.
On a flat sheet enclosure this is automatic. On a folded profile, a corrugated roof panel or a cast housing with a draft angle, it is not. The standard answers are a flat pad welded or bonded around the entry, a sealing washer with a larger outside diameter than the gasket, or moving the entry to a machined face.
Edge distance belongs here too. A hole placed close to a fold has a gasket that overhangs the bend, and a locknut that cannot get a full grip on one side. As a working rule, keep the hole centre far enough from any edge that the whole gasket ring and the full locknut sit on flat material, and check that rule on the drawing rather than at the drill.
Spacing is decided by the spanner, not by the thread. Two M20 glands on a 25 mm pitch have holes that are correctly sized and glands that cannot both be tightened, because the wrench that fits one cannot reach the other. The across-flats dimension on the datasheet is the number to lay out against — for M20 it is 22 mm, and the hexagon needs clearance on both sides before a second entry is placed next to it.
Two more layout rules that survive contact with a workshop. Leave enough room under the panel for the locknut to be turned, not just to exist; a locknut with no swing space gets tightened with a screwdriver and a hammer. And keep the entry away from the door hinge line and the cable duct, because the cable has to leave the gland without a bend radius tighter than the conductor allows.
Existing holes are a constraint, not a reason to accept a bad gland fit. There are four practical recoveries, and only one of them is invisible.
| Problem | Recovery | Trade-off |
|---|---|---|
| Hole too large | Reducing washer plus a larger gland body, or a stepped adapter | Adds a joint and axial length; must keep the gasket bearing face |
| Hole too small | Ream or step-drill to size | Only works if the position and edge distance allow it |
| Hole off position | Weld or plug and re-cut; or fit a larger gland that covers the old hole | Welding disturbs coating; a larger gland changes the cable range |
| Wrong thread in an existing tapped boss | Adapter to the required thread | Adds a component that has to be recorded in the maintenance file |
Whichever route is taken, the entry still has to end up with a flat annulus, a full gasket ring and a thread that the gland can be tightened onto. A recovery that produces a watertight-looking assembly with no compression on the gasket has not recovered anything.
20 to 20.2 mm. Use a 20 mm punch or hole saw and check the result: a hole saw that cuts 20.4 mm leaves the gasket bearing on the very edge of its ring.
You can, but you will have to force the gland in and the coating will be scraped off in the process. The 0.2 mm clearance exists so the thread enters without damage.
Yes, for the same thread. The thread is cut to the same standard, so an M20 nylon gland and an M20 brass gland both need 20 to 20.2 mm. Cable ranges differ between the two, which is a selection issue rather than a hole issue.
Enough for the spanner to turn on both. Lay out from the across-flats dimension plus tool clearance, not from the hole diameter plus a few millimetres.
No, but it changes the wall thickness the gland can work with. A locknut consumes thread length on the far side, so a thick wall plus a locknut can leave no engagement at all.
If you send us the panel drawing with the cable list, we will mark the hole diameter, thread and clearance for each entry before anything is cut. The brass series and the stainless steel series share this hole table, and the installation guide covers what to do once the holes are in.
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